The invention relates generally to charging of rechargeable energy storage systems (e.g., batteries and electric double-layer capacitors and the like) and more particularly to charging storage systems in conditions when a peak line voltage is greater than a storage system voltage due to a low state-of-charge (SOC).
FIG. 1 is a prior art schematic block diagram representative of a voltage converter 100 used in some charging systems installed onboard an electric vehicle to charge a high energy capacity battery assembly 105. In general, circuit 100 describes a boost rectifier for an electric vehicle that includes a set of four transistors (Q1-Q4) having an output coupled to a smoothing capacitor Cbus in parallel with battery assembly 105 and an input coupled to an input filter capacitor Cin and a pair of inductors (L1 and L2). A pair of switches (S1 and S2) couple the inductors to an AC source, the AC source nominally providing 240 volts at about 70 amps. In normal operation, switch S1 and switch S2 are closed and S3 is open, and converter 100 operates in normal boost mode without difficulties.
There are situations wherein the line voltage provides a higher than peak voltage (e.g., +10% greater voltage or about 370 volts (e.g., VRMS*1.1*√{square root over (2)})) and battery assembly 105 is at a lower end of its SOC (e.g., ˜330 volts) that voltage converter 100 cannot charge battery assembly 105 in the normal mode. (Voltage converter 100 cannot charge battery assembly 105 because the described relative voltages results in uncontrollable current flow out of the rectifier into Cbus and into the battery assembly, potentially seriously damaging both of them.)
For this non-standard condition, voltage converter 100 includes resistor Rtrickle and a switch S3. Switch S1 and switch S3 are closed and switch S2 is opened, and Rtrickle then reduces input voltage applied to the boost rectifier. In some implementations, one-half of the input voltage is dropped across Rtrickle, greatly reducing the effective voltage applied to converter 100 (with about 7 amps of charging current available in this trickle charge mode). Voltage converter 100 operates in this non-standard mode until the relative voltage conditions between AC line-in voltage and the voltage level of energy storage assembly 105 is sufficient to reconfigure the converter to normal mode operation (i.e., opening switch S3 and closing switch S2).
While this solution is acceptable in many applications, as storage voltages decrease, it becomes increasingly likely that there is a need for non-standard operation to account for Vbat being lower than the experienced peak voltage from the AC line-in. The solution shown in FIG. 1 is very lossy and because of the greatly reduced current flow (Itickle), it is particularly disadvantageous the more frequently that the solution is employed.
In the case of solutions to be applied to electric vehicles, there are often tight budgets for space, weight and component costs. Conventional methodologies for solving the problem addressed by the trickle mode shown in FIG. 1 include addition of separate converters to handle each special case of relative line voltage/SOC conditions or changing the system to employ an isolated topology.
Adding separate converters is viewed as undesirable because of extra costs and space. The components in the converters are high performance components configured for this high power/high energy operation and are more expensive than many lower power options. An isolated topology often produces a less efficient solution. Thus neither conventional solution is optimal.
A further drawback of the solution shown in FIG. 1 is that there are situations in which a voltage level of the battery assembly is so low that it would be unsafe to attempt to restore function by trickle charging.
What is needed is a voltage converter that is capable of providing high energy to a high performance energy storage assembly for charging the energy storage assembly while efficiently and safely handling conditions of a too “high” line-in voltage relative to a voltage level of the energy storage assembly while also being capable of use in reviving a battery assembly having a very low level state-of-charge.